OK we all like to have decent maps and images of stuff to show our players or just to help us come up with ideas, but a lot of 3d modellers are hard to learn (Tell me about it!) and some can be expensive.
There's a jnice free one (not the full version, but a useful one) called "sketchup 3d" and it has a load of free models available for it as well as tutorials made by users and passed out for free.
Here's where to get Sketchup", the free version: http://www.google.com/sketchup/download/gsu.html
Here's where to get free user tutorials: http://sketchup.google.com/3dwarehouse/cldetails?mid=36e1fa0d054a15eecc725c514c21d975&prevstart=0
And here's where to get free models, including some poular SF models and homemade models as well: http://sketchup.google.com/3dwarehouse/search?q=science+fiction&styp=m&btnG=Search
If you've tried and failed to master some 3d modellers don't feel stupid, some of them are a bitch. Sketchup 3d is supposed to be easy and I'm going to give it a crack myself.
Also, even if you don't make much with it, you can still download free original models to use, as long as you're using them for non commercial stuff.
A really cool thing is whejn you select a model on 3d warehouse you can rotate it in real time to see a full view of it. selects a model from this page and try it: http://sketchup.google.com/3dwarehouse/search?q=science+fiction&styp=m&btnG=Search&start=36